feat: implement proto-gen platform

- Config loader with env var expansion and validation
- SQLite database with tasks and toolchain_versions tables
- Gitea REST API client for repository and branch operations
- K8s client for Job lifecycle management (with noop fallback for local dev)
- REST API: tasks CRUD, retry, plugins, version management
- Worker/scheduler with polling, concurrency control, and job tracking
- Static frontend: generate, tasks, settings pages
- Toolchain images: Go and Node.js Dockerfiles with entrypoint scripts
- Helm chart with deployment, service, configmap, ingress, PVC
- Multi-stage Dockerfile for service
- Unit tests for config, database, gitclient, k8s, api, worker
- E2E tests for idempotency, version switch, failure recovery

internal/static/settings.html

@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html lang="zh-CN">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Settings - Proto-Gen Platform</title>
+    <link rel="stylesheet" href="/style.css">
+</head>
+<body>
+    <nav>
+        <a href="/" class="logo">Proto-Gen</a>
+        <div class="nav-links">
+            <a href="/">Generate</a>
+            <a href="/tasks.html">Tasks</a>
+            <a href="/settings.html" class="active">Settings</a>
+        </div>
+    </nav>
+
+    <main>
+        <h1>Settings</h1>
+        <h2>Toolchain Versions</h2>
+        <div id="pluginsList"></div>
+    </main>
+
+    <script src="/app.js"></script>
+    <script>
+        initSettingsPage();
+    </script>
+</body>
+</html>